Data Protection minor tidying (#31512)

* Remove unused code

Remove unused constant, method and variable.

* Remove array allocations

Use Array.Empty() instead of allocating an empty array.
Use a static array for activation type instead of creating a new one per invocation.

* Rename field

Address PR feedback.

src/DataProtection/DataProtection/src/EphemeralDataProtectionProvider.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Diagnostics;
 using System.Runtime.InteropServices;
-using System.Runtime.Versioning;
 using Microsoft.AspNetCore.Cryptography.Cng;
 using Microsoft.AspNetCore.DataProtection.AuthenticatedEncryption;
 using Microsoft.AspNetCore.DataProtection.AuthenticatedEncryption.ConfigurationModel;
@@ -85,9 +84,6 @@ public EphemeralKeyRing(ILoggerFactory loggerFactory)
  DefaultAuthenticatedEncryptor = GetDefaultEncryptor(loggerFactory);
  }
 
- // Currently hardcoded to a 512-bit KDK.
- private const int NUM_BYTES_IN_KDK = 512 / 8;
-
  public IAuthenticatedEncryptor? DefaultAuthenticatedEncryptor { get; }
 
  public Guid DefaultKeyId { get; } = default(Guid);

src/DataProtection/DataProtection/src/Managed/ManagedAuthenticatedEncryptor.cs

@@ -69,7 +69,7 @@ public ManagedAuthenticatedEncryptor(Secret keyDerivationKey, Func<SymmetricAlgo
 
  private byte[] CreateContextHeader()
  {
- var EMPTY_ARRAY = new byte[0];
+ var EMPTY_ARRAY = Array.Empty<byte>();
  var EMPTY_ARRAY_SEGMENT = new ArraySegment<byte>(EMPTY_ARRAY);
 
  var retVal = new byte[checked(

src/DataProtection/DataProtection/src/SimpleActivator.cs

@@ -2,7 +2,6 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
-using System.Reflection;
 using Microsoft.AspNetCore.DataProtection.Internal;
 
 namespace Microsoft.AspNetCore.DataProtection
@@ -13,6 +12,8 @@ namespace Microsoft.AspNetCore.DataProtection
  /// </summary>
  internal class SimpleActivator : IActivator
  {
+ private static readonly Type[] _serviceProviderTypeArray = { typeof(IServiceProvider) };
+
  /// <summary>
  /// A default <see cref="SimpleActivator"/> whose wrapped <see cref="IServiceProvider"/> is null.
  /// </summary>
@@ -42,7 +43,7 @@ public virtual object CreateInstance(Type expectedBaseType, string implementatio
  }
 
  // If an IServiceProvider was specified or if .ctor() doesn't exist, prefer .ctor(IServiceProvider) [if it exists]
- var ctorWhichTakesServiceProvider = implementationType.GetConstructor(new Type[] { typeof(IServiceProvider) });
+ var ctorWhichTakesServiceProvider = implementationType.GetConstructor(_serviceProviderTypeArray);
  if (ctorWhichTakesServiceProvider != null)
  {
  return ctorWhichTakesServiceProvider.Invoke(new[] { _services });

src/DataProtection/DataProtection/src/XmlEncryption/EncryptedXmlDecryptor.cs

@@ -59,7 +59,6 @@ public XElement Decrypt(XElement encryptedElement)
  // doesn't handle encrypting the root element all that well.
  var xmlDocument = new XmlDocument();
  xmlDocument.Load(new XElement("root", encryptedElement).CreateReader());
- var elementToDecrypt = (XmlElement)xmlDocument.DocumentElement!.FirstChild!;
 
  // Perform the decryption and update the document in-place.
  var encryptedXml = new EncryptedXmlWithCertificateKeys(_options, xmlDocument);
@@ -68,7 +67,7 @@ public XElement Decrypt(XElement encryptedElement)
  encryptedXml.DecryptDocument();
 
  // Strip the <root /> element back off and convert the XmlDocument to an XElement.
- return XElement.Load(xmlDocument.DocumentElement.FirstChild!.CreateNavigator()!.ReadSubtree());
+ return XElement.Load(xmlDocument.DocumentElement!.FirstChild!.CreateNavigator()!.ReadSubtree());
  }
 
  void IInternalEncryptedXmlDecryptor.PerformPreDecryptionSetup(EncryptedXml encryptedXml)

src/Shared/WebEncoders/WebEncoders.cs

@@ -404,22 +404,6 @@ private static int Base64UrlEncode(ReadOnlySpan<byte> input, Span<char> output)
  }
 #endif
 
- private static int GetNumBase64PaddingCharsInString(string str)
- {
- // Assumption: input contains a well-formed base64 string with no whitespace.
-
- // base64 guaranteed have 0 - 2 padding characters.
- if (str[str.Length - 1] == '=')
- {
- if (str[str.Length - 2] == '=')
- {
- return 2;
- }
- return 1;
- }
- return 0;
- }
-
  private static int GetNumBase64PaddingCharsToAddForDecode(int inputLength)
  {
  switch (inputLength % 4)